An internal combustion engine may be used as a motor vehicle drive. The expression “internal combustion engine” encompasses diesel engines, Otto-cycle (e.g., sparking ignition) engines, and also hybrid internal combustion engines, which utilize a hybrid combustion process, and hybrid drives which comprise not only the internal combustion engine, but also an electric machine. The electric machine may be connected in terms of drive to the internal combustion engine and may receive power from the internal combustion engine or, as a switchable auxiliary drive, may additionally output power.
In recent years, there has been a trend in development toward supercharged engines, wherein the economic significance of said engines for the automobile construction industry continues to steadily increase. Supercharging is primarily a method for increasing power in which the air required for the combustion process in the engine is compressed, as a result of which a greater air mass can be fed to each cylinder in each working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In all cases, supercharging leads to an increase in volumetric power output and a more expedient power-to-weight ratio. If the swept volume is reduced, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption and increase the efficiency of the internal combustion engine.
By means of a suitable transmission configuration, it is additionally possible to realize so-called downspeeding, whereby a lower specific fuel consumption is likewise achieved. In the case of downspeeding, use is made of the fact that the specific fuel consumption at low engine speeds is generally lower, in particular in the presence of relatively high loads.
For supercharging, use is often made of an exhaust gas turbocharger, in which a compressor and a turbine are arranged on a same shaft. Hot exhaust gas flow is fed to the turbine and expands in the turbine with a release of energy, which rotates the shaft. The energy released by the exhaust gas flow to the turbine and ultimately to the shaft is used for driving the compressor, which is arranged in an intake system of the engine. The compressor conveys and compresses charge air fed to it, resulting in supercharging of the engine. A charge-air cooler is commonly provided in the intake system downstream of the compressor, by means of which the compressed charge air is cooled before it enters cylinders of the engine. The charge-air cooler lowers the temperature and thereby increases the density of the charge air, such that the charge-air cooler also contributes to increased charging of the cylinders (e.g., with a greater air mass), such as through compression by cooling.
An advantage of an exhaust gas turbocharger in relation to a supercharger, which can be driven by means of an auxiliary drive, is that an exhaust gas turbocharger utilizes the exhaust gas energy of the hot exhaust gases, whereas a supercharger draws the energy required for driving it directly or indirectly from the internal combustion engine and thus reduces the engine efficiency, at least for as long as the drive energy does not originate from an energy recovery source. If the supercharger is not one that can be driven by means of an electric machine (e.g., electrically) a mechanical or kinematic connection for power transmission is generally required between the supercharger and the internal combustion engine, which also influences packaging of the engine system.
An advantage of a supercharger in relation to an exhaust gas turbocharger is that the supercharger can generate, and make available, a desired charge pressure at all times, specifically regardless of the operating state of the internal combustion engine. This applies in particular to a supercharger which can be driven electrically by means of an electric machine, and is therefore independent of the rotational speed of the crankshaft.
In some internal combustion engine systems, one or more intake charging devices may be staged in series or parallel in what may be referred to as a compound boosting configuration. For example, a fast, auxiliary boosting device (e.g., an electric supercharger, or e-booster) may be utilized to increase the transient performance of a slower, primary boosting device (e.g., the exhaust gas turbocharger). As a result, an increase in power may be achieved in all engine speed ranges.
With targeted configuration of the supercharging, it is possible to not only reduce fuel consumption and increase the efficiency of the internal combustion engine, but also reduce exhaust gas emissions. To be able to adhere to future limit values for pollutant emissions, however, further measures are necessary in addition to the supercharging system. As one example, the reduction of nitrogen oxide emissions is of high relevance in particular in diesel engines. Since the formation of nitrogen oxides requires not only an excess of air but rather also high temperatures, one concept for lowering the nitrogen oxide emissions includes developing combustion processes with low combustion temperatures.
As one example, exhaust gas recirculation (EGR) may be used to reduce combustion temperatures, wherein it is possible for the nitrogen oxide emissions to be considerably reduced with increasing exhaust gas recirculation rate. Here, the exhaust gas recirculation rate xEGR is determined as xEGR=mEGR/(mEGR+mair), where mEGR denotes the mass of recirculated exhaust gas and mair denotes the supplied air. To obtain a considerable reduction in nitrogen oxide emissions, high exhaust gas recirculation rates may be used, which may be approximately xEGR≈60% to 70% or more. Such high recirculation rates generally require cooling of the exhaust gas for recirculation.
For example, the internal combustion engine may include an exhaust gas recirculation system that recirculates exhaust gases from downstream of the turbocharger turbine to upstream of the turbocharger compressor via a recirculation line. Said exhaust gas recirculation system is consequently a low-pressure EGR system. In the recirculation line, there are generally provided a cooler and an EGR valve for setting the recirculated exhaust gas flow rate.
An advantage of low-pressure EGR compared with high-pressure EGR, in which the exhaust gas for recirculation is extracted from upstream of the turbine and is no longer available for driving the turbine, is that, regardless of the present recirculation rate, all of the exhaust gas from the internal combustion engine is available at the turbine. A reduced exhaust gas mass flow through the turbine specifically leads to a lower turbine pressure ratio and, thus, a lower charge pressure ratio, which equates to a smaller compressor mass flow. Therefore, the low-pressure EGR system avoids the smaller compressor mass flow.
The exhaust gas which is recirculated via the low-pressure EGR system to the intake system is mixed with fresh air upstream of the turbocharger compressor. The mixture of fresh air and recirculated exhaust gas produced in this way forms the charge air which is supplied to the compressor and compressed, which may be cooled, downstream of the compressor, in a charge-air cooler.
However, the inventors herein have recognized potential issues with such systems. As one example, condensate may form when the exhaust gas recirculation system is active and exhaust gas is introduced into the intake system upstream of the compressor. For example, condensate may form if recirculated hot exhaust gas meets, and is mixed with, cool fresh air. The exhaust gas cools down, whereas a temperature of the fresh air is increased. The temperature of the mixture of fresh air and recirculated exhaust gas, that is to say the charge-air temperature, lies below the exhaust gas temperature of the recirculated exhaust gas. During the course of the cooling of the exhaust gas, components previously contained in the exhaust gas still in gaseous form, such as water, may condense if the dew point temperature of a component of the gaseous charge-air flow is undershot. When condensate formation occurs in the charge-air flow, contaminants in the charge air often form a starting point for the formation of condensate droplets. As another example, condensate can form when hot exhaust gas and/or the charge air impinges on the internal wall of the intake system, as the wall temperature generally lies below the dew point temperature of the relevant gaseous components.
The problem of condensate formation is intensified with increasing recirculation rate because, with the increase of the recirculated exhaust gas flow rate, the fractions of the individual exhaust gas components in the charge air, in particular the fraction of the water contained in the exhaust gas, inevitably increase. In the prior art, therefore, the exhaust gas flow rate recirculated via the low-pressure EGR system is commonly limited in order to prevent or reduce the occurrence of condensation. The required limitation of the low-pressure EGR on the one hand and the high exhaust gas recirculation rates required for a considerable reduction in the nitrogen oxide emissions on the other hand conflict, resulting in reduced reduction of the nitrogen oxide emissions. The problem of condensate formation is also intensified with decreasing ambient temperature because, with the decrease in ambient temperature, the charge-air temperature, that is to say the temperature of the mixture of fresh air and recirculated exhaust gas, decreases, a result of which the charge air can in particular absorb less gaseous water. Here, the recirculated exhaust gas is more intensely cooled.
Condensate and condensate droplets are undesirable and lead to increased noise emissions in the intake system and may degrade the turbocharger compressor. The latter effect is associated with a reduction in efficiency of the compressor.
In one example, the issues described above may be addressed by a system for a supercharged internal combustion engine, comprising: an intake system for the supply of charge air; an exhaust gas discharge system for the discharge of exhaust gas; a turbocharger, including a turbine arranged in the exhaust gas discharge system and a compressor arranged in the intake system; an electrically drivable compressor arranged in the intake system upstream of the compressor of the turbocharger; a compressor-specific throttle element is arranged upstream of the electrically drivable compressor; a bypass line for bypassing said electrically drivable compressor that branches off from the intake system upstream of the electrically drivable compressor, forming a third junction, and opens into the intake system between the electrically drivable compressor and the compressor of the turbocharger, forming a fourth junction; a shut-off element arranged in the bypass line; and an exhaust gas recirculation system, comprising a recirculation line which branches off from the exhaust gas discharge system downstream of the turbine of the turbocharger, the recirculation line bifurcating into a first recirculation branch that includes a first EGR valve and opens into the intake system upstream of the third junction, forming a first junction, and a second recirculation branch that includes a second EGR valve and opens into the intake system between the fourth junction and the compressor of the turbocharger, forming a second junction. In this way, the electrically drivable compressor may be used for heating the fresh air drawn in via the intake system in order to counteract the formation of condensate during the introduction of recirculated exhaust gas.
As one example, the fresh air may be throttled using the throttle element upstream of the electrically drivable compressor. Here, the pressure in the fresh air falls, whereas the temperature in the fresh air remains virtually unchanged. During the subsequent compression in the electrically driven compressor arranged downstream of the throttle element, the pressure in the fresh air is then increased again, and the temperature of the fresh air increases. Fresh air at an elevated temperature is then present downstream of the electrically drivable compressor. If exhaust gas is now introduced into said heated or warmer fresh air, a higher charge-air temperature inevitably also results. For example, the temperature of the mixture of fresh air and recirculated exhaust gas is likewise higher, whereby the charge air can absorb more gaseous water. The recirculated exhaust gas is less intensely cooled, whereby the formation of condensate in the charge-air flow is counteracted. The condensate formation as a result of wall contact is likewise counteracted because the heated fresh air heats the walls of the intake system downstream of the electrically driven compressor. As another example, the exhaust gas recirculation system according to the present disclosure may be used during conditions that would otherwise cause condensate formation, for example, after a cold start or in the presence of low ambient temperatures. In this way, the exhaust gas recirculation system may be used in an extended temperature range, increasing fuel economy and reducing vehicle emissions, while degradation of the turbocharger compressor is prevented.
As another example, if relevant condensate formation is not to be expected, with the throttle element open, the electrically drivable compressor may be actively operated and utilized for generating charge pressure, for example, in order to increase transient operating characteristics of the internal combustion engine. In such an example, exhaust gas recirculation may be provided upstream of the electrically drivable compressor via the first recirculation branch.
The heating of the fresh air according to the present disclosure has further advantageous effects. The heated charge air or fresh air assists the warm-up process of the internal combustion engine, in particular after a cold start, for example by means of an introduction of heat into the walls of the intake system and the combustion chamber walls of the cylinders. Owing to the higher temperature level, the untreated emissions of unburned hydrocarbons and carbon monoxide may decrease.
According to the present disclosure, the electrically drivable compressor is designed as an activatable compressor, which is activated when required. In addition to the use described above, the electrically drivable compressor can, in principle, be used whenever there is a need, including, for example, to assist the exhaust gas turbocharger in compressing the charge air. The electrically drivable compressor may also be used to generate the charge pressure instead of the exhaust gas turbocharger, such as during low loads or during low charge-air flow rates.
According to the present disclosure, the electrically drivable compressor does not necessarily have to be switched off after the heating of the fresh air. The electrically drivable compressor may thus continue to be operated even if no condensate formation is to be expected.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.